This invention relates to a controller which may be described more particularly as a wide range linear impedance controller using junction field-effect transistors as the control element.
In digital telecommunication transmission systems, cable length is too long for end-to-end signal transmission without equalization, amplification, and regeneration at designated points along the cable. Regenerators are required to offset the effects of attenuation, as a function of frequency, in the cable conductors. Typical regenerator circuitry includes an automatic line build-out (ALBO) circuit. The function of the ALBO circuit is to add shaped loss, if required, to the cable section so that the regenerator fixed gain equalizer can compensate for a predetermined loss shape. If the cable section is short, most of the loss shape is provided by the ALBO circuit. For a maximum length cable section, the ALBO circuit has minimal effect on the loss shape.
Required magnitude of the adjustable shaped loss covers a wide range because of variables in the design of any specific transmission line. The connected cable section may vary from short to long. Attenuation per unit length of the transmission medium varies from cable to cable. Ambient temperature varies over a wide range from day-to-day and season-to-season. Since the above-mentioned parameters, such as the length of the cable, the attenuation per unit length of the transmission medium, and the ambient temperature, determine the overall attenuation of the signal, it is desirable to provide an automatically adjustable shaped loss circuit with a wide enough dynamic range to provide the desired shaped loss characteristic, regardless of the input cable length, to the fixed gain equalizer.
In some prior art line regenerators, a string of diodes has been used to provide a variable control impedance for the ALBO circuit in dependence upon the variation of the magnitude of the input signal to the regenerator. Such a string of diodes has an insufficient dynamic range for state-of-the-art transmission systems.
Another drawback to the prior art arrangement is that it requires a fairly substantial magnitude of power supply voltage to achieve the dynamic range needed for linear operation. Linear operation means signal processing with a minimum amount of harmonic distortion. Since the power supply voltage is limited along a transmission line, the number of diodes used in series is limited and results in too little dynamic range for practical purposes.
Another prior art controller includes a single field-effect transistor (FET) for providing a variable impedance controlled by the magnitude of the input signal. The field-effect transistor, however, is inherently nonlinear over a wide impedance range. Part of the operating range is linear or nearly linear while another part of the operating range is very nonlinear. Unfortunately the linear part of the operating range is too limited to provide the wide range linear control required by the regenerators used in state-of-the-art transmission systems.
Thus, there is a problem in designing a wide range linear impedance controller for the regenerators to be used in state-of-the-art transmission systems.